1. Field of the Invention
The invention relates generally to a finishing process for elastomeric polymers. More specifically, the invention pertains to a process for the finish drying of a water-slurried elastomer having residual hydrocarbon volatiles wherein an enclosed, self-cleaning, low-shear dryer is utilized to achieve drying temperatures which are significantly lower than those associated with conventional processes.
Conventional finishing processes for water-slurried elastomers involve presenting the water-slurried elastomer to a number of dewatering/drying extruders, usually two or three, whereby the water contained in the elastomer is removed, and then cooling the dried elastomer in a conveyor prior to baling. All of the aforementioned extruders are characterized by short residence times, with the elastomeric material flowing through each of the extruders within 30 seconds to 2 minutes. Examples of extruders of the foregoing type can be found in U.S. Pat. Nos. 4,538,917; 4,484,878; 4,538,917; 4,446,094; 4,383,764; 4,304,054; 4,226,671; 4,185,057; 4,136,251; 4,124,306; 4,063,718; 4,029,300; 3,993,292; 3,908,013; 3,834,440; 3,684,252 and 3,578,740.
Because the residence times for the respective extruders are so short, extensive mechanical shear and very high drying temperatures must be applied in the extruders in order to dry the elastomeric material to the extent required to achieve acceptable water removal. In the last extruder, for example, a drying temperature of 150.degree. C. to 250.degree. C. is necessary to dry the elastomer, with the actual required temperature depending upon the particular type of elastomer and its water content when introduced into the extruder.
The elastomeric material which exits the last drying extruder in crumb form is extremely hot and porous, frequently containing a large amount of fines. Fines are generated as a result of flashing high temperature and high pressure steam from the rubber crumb at the exit of the drying extruder or internally at the vent section. The steam tears apart the rubber crumb and thus generates fines. In order to reduce fines generation, conventional processes reduce temperature and pressure in the drying extruder to avoid the explosion. In the latter case, however, water cannot be removed completely at the drying extruder, and further drying must be provided in vibrating or non-vibrating conveyors. When the hot, dry crumb is supplied to a conveyor for drying and cooling prior to baling, it poses numerous problems detrimental to the finishing process.
Generally, the cooling conveyor consists of a vibrating conveyor such as a fluidized bed conveyor and the like. The hot and porous condition of the elastomeric crumb results in serious fouling and agglomeration problems in the vibrating conveyor. Additionally, the vibrating conveyor is often operated at high temperatures, typically ranging from 90.degree. C. to 130.degree. C., in order to remove additional moisture from the elastomeric crumb, as discussed above. When utilizing high temperatures to achieve further drying, the negative effects of fouling and agglomeration due to the condition of the elastomer are even further intensified.
Other conventional elastomer finishing processes substitute a non-vibrating belt conveyor for the vibrating conveyor. The non-vibrating conveyors, however, experience the same problems and drawbacks associated with the vibrating conveyors. For both the vibrating and non-vibrating conveyor types, the very hot, porous nature of the extruded elastomeric crumb destabilizes and impairs the quality of the finished product, causing discoloration and contamination. Moreover, fouling of the conveyor equipment by the elastomeric material leads to costly and inconvenient downtimes for cleaning of the conveyor and, indeed, is apt to culminate in even costlier repairs and replacement.
Conventional elastomeric finishing processes have also utilized a pneumatic conveyor between the last drying extruder and the heating/cooling vibrating conveyor or non-vibrating belt conveyor. The pneumatic conveyor is susceptible to all of the problems previously discussed in connection with the vibrating and belt conveyors. Additionally, the pneumatic conveyor is subject to the further limitation of undesirable condensation in the conveyor resulting from the high temperature of the elastomeric crumb.
The present invention addresses and overcomes the deficiencies associated with conventional finishing processes for elastomeric materials resulting from the high temperature and porosity of the material when it exits the last drying extruder. The process of the present invention involves drying the elastomeric material in an enclosed, self-cleaning, no-fouling dryer characterized by low shear, long residence times and reduced drying temperatures. According to the invention, the drying is achieved, not by steam flashing, but in a diffusion mechanism, under gentle shear of a dryer agitator and under vacuum. The gentle shear exposes new crumb surface containing the existing internal moisture, and the vacuum accelerates the molecular diffusion of water through the rubber. In this manner, the rubber crumb will not be formed, and high temperature is not necessary. Therefore, fines generation is avoided.
According to the invention, fines are eliminated at the dryer discharge, all of the drying is completed in the dryer, and the elastomer is in a dense crumb or pellet form when it is discharged from the dryer. Thus, the necessity for further drying is removed, and only cooling is required in the downstream conveyor. The dryer may be utilized further to remove residual volatiles from the elastomer. Further in accordance with the invention, the bulk density of the elastomer can be adjusted by injecting a small amount of nitrogen or air or other gases into the discharging screw located at the discharge end of the dryer.
Volatiles are removed from the dryer through a large vent section that is located so as to evacuate liberated volatiles. Compared to conventional processes, the invention provides a concentrated volatile stream from the vent section, undiluted by drying air or nitrogen, and ready to be treated or recovered. By reducing dilution from added gases, the volatile stream is more efficiently processed for emission reduction, thereby providing a significant advantage in reducing emissions. Additionally, the volatile remaining in the product can be as low as about 0.05% by weight and, typically, about 0.10% by weight.
The downstream cooling conveyor may comprise a pneumatic conveyor with a cooling water jacket, absent the concerns and limitations of condensation, fouling and agglomeration. The vibrating conveyor or the non-vibrating belt conveyor may therefore be eliminated.
The invention contemplates utilizing the dryer in replacement of the last drying extruder of conventional elastomeric finishing processes. Alternatively, the dryer may be utilized in addition to, and subsequent to, the final drying extruder. The rubber is discharged from the dryer through a screw/die/cutter arrangement. Pneumatic conveying is preferably introduced at the die/cutter for further cooling. Refrigerated air and a water jacket may be utilized to achieve cooling to baling temperatures. Because no water vapor is present at the latter stage, condensation is avoided. Furthermore, no steam flashing occurs at this stage and, consequently, no fines are generated. The use of refrigerated air and a water jacket serves further to avoid fouling in the pneumatic conveying system.
2. Description of the Prior Art
It is known in the prior art to provide finishing processes for elastomers wherein the elastomer is dried in one or more extruders and further dried and/or cooled in a conveyor. Examples of such extruders are disclosed in U.S. Pat. Nos. 4,538,917; 4,484,878; 4,446,094; 4,383,764; 4,304,054; 4,185,057; 4,136,251; 4,124,306; 4,063,718; 4,029,300; 3,993,292; 3,980,013; 3,834,440; 3,684,252 and 3,578,740.
The prior art also discloses enclosed, self-cleaning, low-shear dryers in the form of a heated mixer-kneader. Such dryers are essentially characterized by a sequence of heated disc elements mounted axially on a rotary shaft. Fixed scraping elements between the discs act to clean the discs and shaft. Embodiments of dryers possessing the latter characteristics are shown in U.S. Pat. Nos. 4,650,338; 4,039,024; 3,880,407; 3,689,035 and 3,687,422.
U.S. Pat. No. 4,650,338, which issued to List
U.S. Pat. No. 4,650,338, which issued to List et al on Mar. 17, 1987, discloses a mixing and kneading machine having a shaft provided with a plurality of discs interconnected by means of a kneading arm. A second more rapidly rotating cleaning shaft cooperates with the first shaft.
U.S. Pat. No. 4,039,024 of Aug. 2, 1977 to List teaches a heat exchanger including a stirring shaft upon which is mounted a plurality of disc elements. A scraper member is secured to and extends outwardly from the shaft. The forward edge of the scraper and the discs are connected by means of a stirrer bar.
U.S. Pat. No. 3,880,407, which issued on Apr. 29, 1975 to List is directed to a mixer and kneader characterized by shaft-mounted agitator elements having stirrer blades secured thereto. A continuous cleaning action is provided by a plurality of counter elements.
U.S. Pat. No. 3,689,035 to List dated Sept. 5, 1972, shows a mixing kneader apparatus wherein disc members are mounted on a stirrer shaft, and kneading bar members are arranged between the disc members. A second stirrer shaft having stirrer arm members serves to clean the disc members of the first shaft. U.S. Pat. No. 3,687,422, which issued to List on Aug. 29, 1972, shows a similar mixing kneader device.
Additionally, Great Britain Patent Number 2,200,570 to List teaches a kneader-mixer which combines discs, mixing bars and counter paddles, as do European Patents 289,647 and 274,668 to List.
The kneading/mixing devices taught by the prior art cited and discussed above are enclosed, self-cleaning devices. The prior art fails to teach or suggest the use of a device of this type as a dryer in an elastomer finishing process to achieve low temperature drying of the elastomer under low shear conditions, and to remove residual volatiles and fines.